Ella focused post-canon long post ahead:
I like the idea of Tadius having to reign Ella in when she has a mean streak towards particular peasants who had perpetuated her abuse in the past. The peasants who verbally and physically taunted/abused her. The people who disregarded her pleas and distress, writing it off as madness, violently sending her back to her abusers just to get her out of their sight. Because her circumstance and uncleanliness was uncomfortable to be around. Maybe she can't stand to be cordial or impartial to the very same people who she watched drag her mother into the square. Or maybe, at her lowest, she says some unpleasant things about all peasantry, flippantly stereotypes them in a moment of temper. Let's her trauma blossom into rage, into a simmering desire (not necessarily action, mind you) for petty cruelty.
We see her deconstruct a lot of her (admittedly not crazy large) bias against peasant and servantry in the show; this being easier for her to accomplish because she was forced into servitude and squalor herself. For example, one of the first things she jabs at Tadius with is his rank as a servant. She establishes her status and her power over his well-being, potentially his life (and this is important for us to see for her character! this is the first time we see her so confident in her dialog in the show. she is finally reclaiming the autonomy, power, and safety she lacked for potential years, even if this isn't "perfect victim" or kind behavior). This threat could easily have been genuinely fear inducing for Tadius if he hadn't perceived her comment as banter/wit (or he, at the very least, thought she couldn't do anything to him as the prince's right hand servant because she's not from a reputable house). But later, we see her call Tadius "a good man" instead of "a good servant", signifying her growth. She acknowledges where she went wrong in the past and asserts Tadius's position as an equal. Textbook character development.
She also originally vows to the fairy godmother to take vengeance only on those who deserve it, who have taken everything from her and seek to destroy more. She makes clear that her blood lust is for specifically her step family (edit: if Justine and Lucy represent Ella's core values and dreams then Justine establishing kindness to those socially lower to them as an admirable trait makes a whole lotta sense for Ellas character going foward). Hell, she is said to be a good queen by the whole kingdom, implying the peasants favor her rule after she's gained it. She is known by all for her kindness and love which is carried out just as fiercely as her justice.
Does this mean she doesn't have low points? Does this mean that at her lowest Tadius doesn't need to council her and advice her against her (valid, but potentially unfair) anger? That she doesn't let her new found power make it so that, at times, she speaks down to those she deems morally lesser (and could that perception of who is lesser, by this measurement, be influenced by class bias/her past peasant inflicted abuse)? Ella's main character motivation is vengeance in the show, even if her morals are steadfast throughout, I think it would be an interesting conflict between Ella's own ethic values vs. her (again, valid) trauma-induced feelings/vitriol, and how that conflict influences her decisions not only as queen, but interpersonally.
This could also lead to some great tension between Ella and Tadius (who are at this point good friends, partners, or at the VERY least close coworkers). With some communication Tadius would be given better insight into Ella's past, interworkings, and decision making. It would also grant Ella an outlet to talk about all this awful shit that happened to her and how it comes back to affect and shape her presently.
Not to mention Tadius's own trauma regarding aristocrats who belittle their subjects. Maybe at her most cruel (aka in a very bad emotional state) Tadius becomes clinical and emotionally disconnected, just as he was with the prince? Unconsciously or not, sinks back into giving stony irritated advice and doesn't talk to Ella in their normal affable manner for the rest of the day, or longer. Maybe it takes them a while to get to the point of healthy communication because of both of their past isolation (ella is purposely isolated by her supposed "madness" and has no one to talk to, she is overjoyed to vent to a literal frog + no one recognized tadius's wit before ella, implying he is not close enough with anyone for them to know who he is personally) makes it difficult for them to talk in a productive manner at first.
(I do not mean to say this is the only interpretation of Ella's character or the headcanons you should adopt post-cc. Maybe Ella never let's these internal biases (that I've picked up on) influence how she treats her subjects/how she rules. Or, shes developed and grown entirely past them. Maybe Ella and Tadius never have this kind of conflict between them. This is also tackling a very specific negative trait I enjoy exploring, which I personally could see presenting itself in Ella post-canon. I love her. I don't doubt she is a great queen who rules with the working class's benefit, along with the rich's detriment, in mind (in fact, I definitely like to believe she does). I just really like characters having character flaws, even if they are a good person, even if they are the hero, and even past their happily ever after)
